1. Technical Field of the Invention
This invention relates to a light emitting device such as a light emitting diode (LED) and a laser diode (LD), a photodetector such as a solar cell and an optical sensor, and other nitride semiconductor devices used for electrical devices, for example, a transistor and a power device (which is expressed in the. formula, for instance, InXAlYGa1−X−YN, 0≦X, 0≦Y, X+Y≦1).
2. Description of Related Art
A nitride semiconductor device has been practically developed for use of a high luminous blue and pure green LED to fabricate light sources of a full color LED display, a traffic signal, and an image scanner. The LED device basically comprises a substrate of sapphire, a buffer layer made of GaN, an n-contact layer made of GaN doped with Si, an active layer made of a single quantum well (SQW) structure of InGaN or made of a multiple quantum well (MQW) structure containing InGaN, a p-cladding layer made of AlGaN doped with Mg, and a p-contact layer made of CaN doped with Mg, in which those layers are successively formed on the substrate. The LED device has an excellent opto-electronic characteristics, for example, the blue LED has a peak wavelength of 450 nm, a luminous intensity of 5 mW, and an external quantum efficiency of 9.1%, and the green LED has the peak wavelength of 520 nm, the luminous intensity of 3 mW, and the external quantum efficiency of 6.3%. at the forward current of 20 mA.
Since the multiple quantum well structure has a plurality of mini-bands, each of which emits light efficiently even with a small current, it is expected that the device characteristics is improved, for example, the LED device with the active layer of the multiple quantum well structure characteristics has the luminous intensity greater than that with of the single quantum well structure.
JP10-135514, A, for example, describes the LED device with an active layer of the multiple quantum well structure, which includes a light emitting layer with a barrier layer of undoped GaN and a well layer of undoped InGaN, and also includes cladding layers having bandgap greater than that of the barrier layer of the active layer, in order to improve the luminous efficiency and a luminous intensity.
However the luminous intensity of the conventional LED device is not enough for use as a light source of a illumination lamp and/or a outside display exposed to direct sunshine. It has been long felt needed that the light emitting device having an active layer of quantum well structure will be improved in its luminous intensity, but such a LED device with higher luminous intensity has not yet been available.
Also, the device made of nitride semiconductor has a layer structure, which may be inherently be weak against the electrostatic voltage. Thus, the device of nitride semiconductor may be easily damaged even by the electrostatic voltage of 100V which is much lower than that people can feel. There are substantial risks of damaged device characteristics in handling the device, for example, taking it out of an antistatic bag, and assembling it to a product. Therefore, the electrostatic withstanding voltage of the device has been desirably improved reducing the aforementioned risks, thereby enhancing the reliability of the nitride semiconductor device.